1. Field of the invention
The present invention relates, in general, to reflection mirrors which reflect visible rays and transmit infrared rays from a light source. The above-described reflection mirror is generally used with a lamp, which may radiate a cool-light to such as displayed articles.
2. Description of the prior art
In conventional lamps including a reflection mirror, a glass base is disposed at the backside of the lamp. The inner surface of the glass base is provided with an interference filter for reflecting a large amount of visible rays and for transmitting a large quantity of infrared rays radiated from the lamp. Therefore, the lamp radiates the visible rays containing a small amount of infrared rays, as a cool-light. Such a reflection mirror is called a cold mirror.
The above-described interference filter includes a high refractive index layer made of titanium dioxide, and a low refractive index layer made of silica alternatively stacked one on the other for a total of ten or fifteen layers stocked on the glass base. The interference filter reflects light in a specific wavelength range and transmits light in another range by interference of the light. The reflection wavelength range and the transmition wavelength range are determined by the optical thickness of each reflective layer.
The interference filter with the above-described construction has a low light absorption factor over the entire wavelength range. However, the visible ray reflectance is low in this conventional reflection mirror because of one interference filter arranged on the inner surface of the glass base.
In a sound example of the reflection mirror, a pair of interference filters have the same characteristics one as the other, are respectively arranged on opposite surfaces of a glass base for improving the reflectance of the one example described above. The light transmitting through one interference filter which is disposed on the one surface of the glass base is reflected toward the one interference filter at a prescribed rate by the other interference filter disposed on the other surface of the glass base. Finally, the light transmits from each interference filter while the light reciprocates between interference filters by reflection. This phenomenon is referred to as a interference between the repeating reflection filters in this specification.
In the second example of the reflection mirror described above, the visible ray reflectance is improved. However, since infrared rays also are reflected by the interference filters by the repeating reflection, the infrared rays transmitting toward the lamp increases. Therefore, the infrared rays transmission factor is adversely affected.